A crankcase lubricant is an oil used for general lubrication in an internal combustion engine where an oil sump is situated generally below the crankshaft of the engine and to which circulated oil returns. To reduce the energy and fuel consumption requirements of the engine, there is a need for crankcase lubricants that reduce the overall friction of the engine. Reducing friction losses in an engine contributes significantly to improving fuel economy performance and fuel economy retention properties. It has long been known to use friction modifiers to obtain improved friction performance. However, the effect of such friction modifiers may not be fully realised due to preferred absorption of other additives on moving surfaces.
Oil-soluble molybdenum containing additives may be used for their friction reducing properties. Examples of patent applications which refer to oil-soluble molybdenum additives for lubricating oil compositions include U.S. Pat. Nos. 4,164,473; 4,176,073; 4,176,074; 4,192,757; 4,248,720; 4,201,683; 4,289,635 and 4,479,883.
In particular, International patent application No. WO 00/71649 discloses the use of oil-soluble molybdenum compounds at levels providing from 10-350 ppm molybdenum to the lubricating oil. When used in combination with a particular zinc dialkyldithiophosphate, a particular base stock composition and a supplementary friction modifier, it is said that enhanced fuel economy and fuel economy retention can be obtained, despite the relatively low amount of molybdenum present in the lubricating oil composition.
U.S. Pat. No. 6,423,671 ('671) relates to lubricating compositions with improved frictional characteristics which translates into improved fuel economy when the compositions are used in internal combustion engines. In particular, '671 relates to lubricant compositions containing organo-molybdenum compounds together with zinc salts, metal-containing detergents and ashless friction modifiers (referred to as surfactants). '671 states that molybdenum compounds can improve frictional characteristics but that their effect is not fully realised in the above particular compositions because of preferred absorption on moving surfaces of the non-molybdenum polar components. This competition for absorption of polar components results, for example, in a tendency for detergents to be absorbed more readily than molybdenum compounds. '671 meets this problem by using dispersants to form a first semi-package with the aforementioned non-molybdenum polar components, the semi-package being made by mixing and heating the components, for example at about 90° C. for about 1-3 hours. The molybdenum component is provided in a second semi-package, and the first and second semi-packages added to an oil of lubricating viscosity.
A problem with the approach described in '671 is that it requires additional processing steps, particularly the preparation of the first semi-package. The problem of competition for absorption has also been addressed in a different way in International patent application No. WO 06/89799 by employing a detergent system of low metal ratio in a lubricating oil composition of low total base number (TBN).
EP 2,650,349A relates to lubricating oil compositions with improved frictional characteristics, fuel economy and fuel economy retention performance. The lubricating oil compositions comprise a molybdenum friction modifier in combination with a polymeric friction modifier that is the reaction product of a functionalised polyolefin, a polyether, a polyol and a monocarboxylic acid chain terminating group.
Fuel economy tests are becoming more closely aligned with engine operations and so fuel economy performance is critical in all temperature regimes including the low temperatures (e.g. ambient temperature (20° C.) to below 0° C.) present at engine start up. Accordingly, there is a need for crankcase lubricants which exhibit desirable friction characteristics thereby reducing friction losses in an engine and improving fuel economy and fuel economy retention performance, particularly fuel economy and fuel economy retention performance at low temperatures present at engine start up.